1. Field of the Invention
The present invention relates to an apparatus and method for heating an ink jet printhead, and more particularly, to an apparatus and method for heating a substrate of an ink jet printhead using pass transistors.
2. Description of the Prior Art
An ink jet printer is a device that produces images on paper by firing precisely sized droplets of ink at precisely defined positions. Ink jet printers form droplets of ink in different ways. The most popular technique used in the art is the bubble jet. In a bubble jet printer, small resistors or heaters are energized to create heat that vaporizes ink to from a bubble. The expansion that creates the bubble causes a droplet of ink to form and eject from the printhead. A typical bubble jet printhead has 64 or 128 or more nozzles, and all of them can be energized to fire a droplet of ink simultaneously or individually.
Quality of the printed images formed by the droplets fired is a function of the printed spot size. Because the size of the printed spots, among other factors, is dependant on the mass of the individual droplets, better control of the drop mass is desired to improve image quality.
The mass of the ejected droplet is, in turn, a strong function of the ink temperature. Temperature relates to the thermal energy in the ink and the size of the vapor bubble that drives the ink from the firing chamber. In addition, temperature affects the viscosity, which in turn also affects the mass of a drop because of viscous losses in the firing chamber.
Attempts have been made at controlling the temperature of a thermal printhead for the purpose of controlling drop mass and thereby to control spot size and image quality. One technique includes using the thermal drop forming system to heat the printhead when it is not being used to form drops.
Currently there are two approaches to heat the printhead for the purpose of temperature control. The first approach is to use the existing heating resistors to also heat the substrate. In this approach, reduced energy pulses are applied to the drop-creation resistors or heaters. The reduced energy pulses do not contain enough energy to cause bubble nucleation and growth, but are sufficient to increase the temperature of the printhead. A potential drawback of using the active heating resistors to maintain the substrate temperature is the added workload to an already highly stressed, highly cycled component of the printer, which increases the probability of failure.
The second approach is to use separate substrate heaters. These substrate heaters are large area devices. They can be connected to a separate power source. Alternatively, they can be driven by a single power source that also provides power to the drop forming heaters during margin operations of the printer as disclosed in U.S. Pat. No. 5,734,392, which is incorporated herein by reference for the purpose of providing background information only. However, using a separate power source increases production cost while using a single power source does not provide a continuous substrate heating mechanism.
Therefore, the need still exists in the art for alternative printhead heating mechanism so as to better control the temperature of a thermal ink printhead.